Surveillance for Ceratomyxa shasta in the Puget Sound watershed, Washington.

Discovery of fish exhibiting clinical signs of ceratomyxosis in Washington State prompted concern over the potential impact of the myxozoan parasite Ceratomyxa shasta on native stocks of steelhead Oncorhynchus mykiss (anadromous rainbow trout). To investigate these concerns, a survey of 16 freshwater systems within the Puget Sound watershed, including Lake Washington, was conducted by sentinel exposure of susceptible fish (cutthroat trout O. clarkii and rainbow trout). Fish were exposed for 7 d during September 2003 and May 2004 and then were returned to a holding facility for monitoring of disease signs. Mortality caused by the parasite occurred only in the exposure group held at the University of Washington Hatchery, which receives its water from Portage Bay of Lake Washington. Fish from all other sites were negative for C. shasta, both visually and by polymerase chain reaction (PCR) assay, except for a single fish held at the Tumwater Falls Hatchery in September 2003. A single deformed spore was detected in that fish, but infection could not be confirmed by PCR and the parasite was not detected from any other fish held at that site during either the September or the May exposure. From these results, we conclude that C. shasta is not likely to have contributed significantly to the decline of steelhead populations throughout Puget Sound.